When the air is humid and charged with aqueous vapour, a similar

cooling effect also takes place, but in a slightly different way. Air

charged with aqueous vapour is a good absorber of radiant heat, but

it can only absorb those rays which agree with it in _period_. It so

happens that rays from snow and ice are, of all others, those which it

absorbs best. The humid air will absorb the total radiation from the

snow and ice, but it will allow the greater part of, if not nearly all,

the sun’s rays to pass unabsorbed. But during the day, when the sun is

shining, the radiation from the snow and ice to the air is negative;

that is, the snow and ice cool the air by radiation. The result is, the

air is cooled by radiation from the snow and ice (or rather, we should

say, _to_ the snow and ice) more rapidly than it is heated by the sun;

and, as a consequence, in a country like Greenland, covered with an

icy mantle, the temperature of the air, even during summer, seldom

rises above the freezing-point. Snow is a good reflector, but as simple

reflection does not change the character of the rays they would not be

absorbed by the air, but would pass into stellar space.

Were it not for the ice, the summers of North Greenland, owing to the

continuance of the sun above the horizon, would be as warm as those of

England; but, instead of this, the Greenland summers are colder than

our winters. Cover India with an ice sheet, and its summers would be

colder than those of England.

_Second._ Another cause of the cooling effect is that the rays which

fall on snow and ice are to a great extent reflected back into

space.[32] But those that are not reflected, but absorbed, do not raise

the temperature, for they disappear in the mechanical work of melting

the ice. The latent heat of ice is about 142° F.; consequently in the

melting of every pound of ice a quantity of heat sufficient to raise

one pound of water 142° disappears, and is completely lost, so far

as temperature is concerned. This quantity of heat is consumed, not

in raising the temperature of the ice, but in the mechanical work of

tearing the molecules separate against the forces of cohesion binding

them together into the solid form. No matter what the intensity of the

sun’s heat may be, the surface of the ground will remain permanently at

32° so long as the snow and ice continue unmelted. [**P1:missing page

number]

_Third._ Snow and ice lower the temperature by chilling the air and

condensing the vapour into thick fogs. The great strength of the sun’s

rays during summer, due to his nearness at that season, would, in the

first place, tend to produce an increased amount of evaporation. But

the presence of snow-clad mountains and an icy sea would chill the

atmosphere and condense the vapour into thick fogs. The thick fogs

and cloudy sky would effectually prevent the sun’s rays from reaching

the earth, and the snow, in consequence, would remain unmelted during

the entire summer. In fact, we have this very condition of things

exemplified in some of the islands of the Southern Ocean at the present

day. Sandwich Land, which is in the same parallel of latitude as the

north of Scotland, is covered with ice and snow the entire summer;

and in the island of South Georgia, which is in the same parallel

as the centre of England, the perpetual snow descends to the very

sea-beach. The following is Captain Cook’s description of this dismal

place:—“We thought it very extraordinary,” he says, “that an island

between the latitudes of 54° and 55° should, in the very height of

summer, be almost wholly covered with frozen snow, in some places many

fathoms deep.... The head of the bay was terminated by ice-cliffs of

considerable height; pieces of which were continually breaking off,

which made a noise like a cannon. Nor were the interior parts of the

country less horrible. The savage rocks raised their lofty summits till

lost in the clouds, and valleys were covered with seemingly perpetual

snow. Not a tree nor a shrub of any size were to be seen. The only

signs of vegetation were a strong-bladed grass growing in tufts, wild

burnet, and a plant-like moss seen on the rocks.... We are inclined to

think that the interior parts, on account of their elevation, never

enjoy heat enough to melt the snow in such quantities as to produce

a river, nor did we find even a stream of fresh water on the whole

coast.”[33]

Captain Sir James Ross found the perpetual snow at the sea-level at

Admiralty Inlet, South Shetland, in lat. 64°; and while near this

place the thermometer in the very middle of summer fell at night to

23° F.; and so rapidly was the young ice forming around the ship that

he began, he says, “to have serious apprehensions of the ships being

frozen in.”[34] At the comparatively low latitude of 59° S., in long.

171° E. (the corresponding latitude of our Orkney Islands), snow was

falling on the longest day, and the surface of the sea at 32°.[35] And

during the month of February (the month corresponding to August in our

hemisphere) there were only three days in which they were not assailed

by snow-showers.[36]

In the Straits of Magellan, in 53° S. lat., where the direct heat of

the sun ought to be as great as in the centre of England, MM. Churrca

and Galcano have seen snow fall in the middle of summer; and though the

day was eighteen hours long, the thermometer seldom rose above 42° or

44°, and never above 51°.[37]

This rigorous condition of climate chiefly results from the rays

of the sun being intercepted by the dense fogs which envelope those

regions during the entire summer; and the fogs again are due to the

air being chilled by the presence of the snow-clad mountains and the

immense masses of floating ice which come from the antarctic seas. The

reduction of the sun’s heat and lengthening of the winter, which would

take place when the eccentricity is near to its superior limit and the

winter in aphelion, would in this country produce a state of things

perhaps as bad as, if not worse than, that which at present exists in

South Georgia and South Shetland.

If we turn our attention to the polar regions, we shall find that

the cooling effects of snow and ice are even still more marked. The

coldness of the summers in polar regions is owing almost solely to this

cause. Captain Scoresby states that, in regard to the arctic regions,

the general obscurity of the atmosphere arising from fogs or clouds is

such that the sun is frequently invisible during several successive

days. At such times, when the sun is near the northern tropic, there is

scarcely any sensible quantity of light from noon till midnight.[38]

“And snow,” he says, “is so common in the arctic regions, that it may

be boldly stated that in nine days out of ten during the months of

April, May, and June more or less falls.”[39]

On the north side of Hudson’s Bay, for example, where the quantity of

floating ice during summer is enormous, and dense fogs prevail, the

mean temperature of June does not rise above the freezing-point, being

actually 13°·5 below the normal temperature; while in some parts of

Asia under the same latitude, where there is comparatively little ice,

the mean temperature of June is as high as 60°.

The mean temperature of Van Rensselaer Harbour, in lat. 78° 37′ N.,

long. 70° 53′ W., was accurately determined from hourly observations

made day and night over a period of two years by Dr. Kane. It was found

to be as follows:—

°

Winter −28·59

Spring −10·59

Summer +33·38

Autumn - 4·03

But although the quantity of heat received from the sun at that

latitude ought to have been greater during the summer than in

England,[40] yet nevertheless the temperature is only 1°·38 above the

freezing-point.

The temperature of Port Bowen, lat. 73° 14′ N., was found to be as

follows:—

°

Winter −25·09

Spring - 5·77

Summer +34·40

Autumn +10·58

Here the summer is only 2°·4 above the freezing-point.

The condition of things in the antarctic regions is even still worse

than in the arctic. Captain Sir James Ross, when between lat. 66° S.

and 77° 5′ S., during the months of January and February, 1841, found

the mean temperature to be only 26°·5; and there were only two days

when it rose even to the freezing-point. When near the ice-barrier on

the 8th of February, 1841, a season of the year equivalent to August

in England, he had the thermometer at 12° at noon; and so rapidly was

the young ice forming around the ships, that it was with difficulty

that he escaped being frozen in for the winter. “Three days later,”

he says, “the thick falling snow prevented our seeing to any distance

before us; the waves as they broke over the ships froze as they fell

on the decks and rigging, and covered our clothes with a thick coating

of ice.”[41] On visiting the barrier next year about the same season,

he again ran the risk of being frozen in. He states that the surface

of the sea presented one unbroken sheet of young ice as far as the eye

could discover from the masthead.

Lieutenant Wilkes, of the American Exploring Expedition, says that the

temperature they experienced in the antarctic regions surprised him,

for they seldom, if ever, had it above 30°, even at midday. Captain

Nares, when in latitude 64°S., between the 13th and 25th February last

(1874), found the mean temperature of the air to be 31°·5; a lower

temperature than is met with in the arctic regions, in August, ten

degrees nearer the pole.[42]

These extraordinarily low temperatures during summer, which we have

just been detailing, were due solely to the presence of snow and ice.

In South Georgia, Sandwich Land, and some other places which we have

noticed, the summers ought to be about as warm as those of England; yet

to such an extent is the air cooled by means of floating ice coming

from the antarctic regions, and the rays of the sun enfeebled by the

dense fogs which prevail, that there is actually not heat sufficient

even in the very middle of summer to melt the snow lying on the

sea-beach.

We read with astonishment that a country in the latitude of England

should in the very middle of summer be covered with snow down to the

sea-shore—the thermometer seldom rising much above the freezing-point.

But we do not consider it so surprising that the summer temperature of

the polar regions should be low, for we are accustomed to regard a low

temperature as the normal condition of things there. We are, however,

mistaken if we suppose that the influence of ice on climate is less

marked at the poles than at such places as South Georgia or Sandwich

Land.

It is true that a low summer temperature is the normal state of

matters in very high latitudes, but it is so only in consequence of

the perpetual presence of snow and ice. When we speak of the normal

temperature of a place we mean, of course, as we have already seen,

the normal temperature under the present condition of things. But

were the ice removed from those regions, our present Tables of normal

summer temperature would be valueless. These Tables give us the normal

June temperature while the ice remains, but they do not afford us the

least idea as to what that temperature would be were the ice removed.

The mere removal of the ice, all things else remaining the same, would

raise the summer temperature enormously. The actual June temperature of

Melville Island, for example, is 37°, and Port Franklin, Nova Zembla,

36°·5; but were the ice removed from the arctic regions, we should

then find that the summer temperature of those places would be about

as high as that of England. This will be evident from the following

considerations:—

The temperature of a place, other things being equal, is proportionate

to the quantity of heat received from the sun. If Greenland receives

per given surface as much heat from the sun as England, its temperature

ought to be as high as that of England. Now, from May 10 till August

3, a period of eighty-five days, the quantity of heat received from

the sun in consequence of his remaining above the horizon is actually

greater at the north pole than at the equator.

Column II. of the following Table, calculated by Mr. Meech,[43]

represents the quantity of heat received from the sun on the 15th of

June at every 10° of latitude. To simplify the Table, I have taken 100

as the unit quantity received at the equator on that day instead of the

unit adopted by Mr. Meech:—

+-----------+---------+-----------+-------------+

| | I. | II. | III. |

| | | | |

| |Latitude.| Quantity | June |

| | | of heat. | temperature.|

+-----------+---------+-----------+-------------+

| | ° | | ° |

|Equator | 0 | 100 | 80·0 |

| | 10 | 111 | 81·1 |

| | 20 | 118 | 81·1 |

| | 30 | 123 | 77·3 |

| | 40 | 125 | 68·0 |

| | 50 | 125 | 58·8 |

| | 60 | 123 | 51·4 |

| | 70 | 127 | 39·2 |

| | 80 | 133 | 30·2 |

|North Pole | 90 | 136 | 27·4 |

+-----------+---------+-----------+-------------+

The calculations are, of course, made upon the supposition that the

quantity of rays cut off in passing through the atmosphere is the

same at the poles as at the equator, which, as we know, is not exactly

the case. But, notwithstanding the extra loss of solar heat in high

latitudes caused by the greater amount of rays that are cut off, still,

if the temperature of the arctic summers were at all proportionate to

the quantity of heat received from the sun, it ought to be very much

higher than it actually is. Column III. represents the actual mean June

temperature, according to Prof. Dove, at the corresponding latitudes.

A comparison of these two columns will show the very great deficiency

of temperature in high latitudes during summer. At the equator, for

example, the quantity of heat received is represented by 100 and the

temperature 80°; while at the pole the temperature is only 27°·4,

although the amount of heat received is 136. This low temperature

during summer, from what has been already shown, is due chiefly to the

presence of snow and ice. If by some means or other we could remove

the snow and ice from the arctic regions, they would then enjoy a

temperate, if not a hot, summer. In Greenland, as we have already seen,

snow falls even in the very middle of summer, more or less, nine days

out of ten; but remove the snow from the northern hemisphere, and a

snow-shower in Greenland during summer would be as great a rarity as it

would be on the plains of India.

Other things being equal, the quantity of solar heat received in

Greenland during summer is considerably greater than in England.

Consequently, were it not for snow and ice, it would enjoy as warm a

climate during summer as that of England. Conversely, let the polar

snow and ice extend to the latitude of England, and the summers of that

country would be as cold as those of Greenland. Our summers would then

be as cold as our winters are at present, and snow in the very middle

of summer would perhaps be as common as rain.

_Mr. Murphy’s Theory._—In a paper read before the Geological Society

by Mr. Murphy[44] he admits that the glacial climate was due to an

increase of eccentricity, but maintains in opposition to me that the

glaciated hemisphere must be that in which the _summer_ occurs in

_aphelion_ during the greatest eccentricity of the earth’s orbit.

I fear that Mr. Murphy must be resting his theory on the mistaken idea

that a summer in aphelion ought to melt less snow and ice than one in

perihelion. It is quite true that the longer summer in aphelion—other

things being equal—is colder than the shorter one in perihelion, but

the quantity of heat received from the sun is the same in both cases.

Consequently the quantity of snow and ice melted ought also to be the

same; for the amount melted is in proportion to the quantity of energy

in the form of heat received.

It is true that with us at present less snow and ice are melted during

a cold summer than during a warm one. But this is not a case in point,

for during a cold summer we have less heat than during a warm summer,

the length of both being the same. The coldness of the summers in

this case is owing chiefly to a portion of the heat which we ought to

receive from the sun being cut off by some obstructing cause.

The reason why we have so little snow, and consequently so little ice,

in temperate regions, is not, as Mr. Murphy seems to suppose, that

the heat of summer melts it all, but that there is so little to melt.

And the reason why we have so little to melt is that, owing to the

warmth of our winters, we have generally rain instead of snow. But

if you increase the eccentricity very much, and place the winter in

perihelion, we should probably have no snow whatever, and, as far as

glaciation is concerned, it would then matter very little what sort of

summer we had.

But it is not correct to say that the perihelion summer of the glacial

epoch must have been hot. There are physical reasons, as we have just

seen, which go to prove that, notwithstanding the nearness of the sun

at that season, the temperature would seldom, if ever, rise much above

the freezing-point.

Besides, Mr. Murphy overlooks the fact that the nearness of the sun

during summer was nearly as essential to the production of the ice, as

we shall shortly see, as his great distance during winter.

We must now proceed to the consideration of an agency which is brought

into operation by the foregoing condition of things, an agency far

more potent than any which has yet come under our notice, viz., the

_Deflection of Ocean-currents_.

_Deflection of Ocean-currents the chief Cause of secular Changes

of Climate._—The enormous extent to which the thermal condition of

the globe is affected by ocean-currents seems to cast new light on

the mystery of geological climate. What, for example, would be the

condition of Europe were the Gulf-stream stopped, and the Atlantic thus

deprived of one-fifth of the absolute amount of heat which it is now

receiving above what it has in virtue of the temperature of space? If

the results just arrived at be at all justifiable, it follows that the

stoppage of the stream would lower the temperature of northern Europe

to an extent that would induce a condition of climate as severe as that

of North Greenland; and were the warm currents of the North Pacific

also at the same time to be stopped, the northern hemisphere would

assuredly be subjected to a state of general glaciation.

Suppose also that the warm currents, having been withdrawn from the

northern hemisphere, should flow into the Southern Ocean: what then

would be the condition of the southern hemisphere? Such a transference

of heat would raise the temperature of the latter hemisphere about

as much as it would lower the temperature of the former. It would

consequently raise the mean temperature of the antarctic regions much

above the freezing-point, and the ice under which those regions are

at present buried would, to a great extent at least, disappear. The

northern hemisphere, thus deprived of the heat from the equator, would

be under a condition of things similar to that which prevailed during

the glacial epoch; while the other hemisphere, receiving the heat from

the equator, would be under a condition of climate similar to what we

know prevailed in the northern hemisphere during a part of the Upper

Miocene period, when North Greenland enjoyed a climate as mild as that

of England at the present day.

This is no mere picture of the imagination, no mere hypothesis devised

to meet a difficult case; for if what has already been stated be not

completely erroneous, all this follows as a necessary consequence from

physical principles. If the warm currents of the equatorial regions

be all deflected into one hemisphere, such must be the condition of

things. How then do the agencies which we have been considering deflect

ocean-currents?

_How the foregoing Causes deflect Ocean-currents._—A high condition

of eccentricity tends, we have seen, to produce an accumulation of

snow and ice on the hemisphere whose winters occur in aphelion. This

accumulation tends in turn to lower the summer temperature, to cut

off the sun’s rays, and so to retard the melting of the snow. In

short, it tends to produce on that hemisphere a state of glaciation.

Exactly opposite effects take place on the other hemisphere, which

has its winter in perihelion. There the shortness of the winters and

the highness of the temperature, owing to the sun’s nearness, combine

to prevent the accumulation of snow. The general result is that the

one hemisphere is cooled and the other heated. This state of things

now brings into play the agencies which lead to the deflection of the

Gulf-stream and other great ocean-currents.

Owing to the great difference between the temperature of the equator

and the poles, there is a constant flow of air from the poles to the

equator. It is to this that the trade-winds owe their existence. Now as

the strength of these winds, as a general rule, will depend upon the

difference of temperature that may exist between the equator and higher

latitudes, it follows that the trades on the cold hemisphere will be

stronger than those on the warm. When the polar and temperate regions

of the one hemisphere are covered to a large extent with snow and ice,

the air, as we have just seen, is kept almost at the freezing-point

during both summer and winter. The trades on that hemisphere will, of

necessity, be exceedingly powerful; while on the other hemisphere,

where there is comparatively little snow and ice, and the air is warm,

the trades will, as a consequence, be weak. Suppose now the northern

hemisphere to be the cold one. The north-east trade-winds of this

hemisphere will far exceed in strength the south-east trade-winds of

the southern hemisphere. The _median-line_ between the trades will

consequently lie to a very considerable distance to the south of

the equator. We have a good example of this at the present day. The

difference of temperature between the two hemispheres at present is

but trifling to what it would be in the case under consideration; yet

we find that the south-east trades of the Atlantic blow with greater

force than the north-east trades, and the result is that the south-east

trades sometimes extend to 10° or 15° N. lat., whereas the north-east

trades seldom blow south of the equator. The effect of the northern

trades blowing across the equator to a great distance will be to impel

the warm water of the tropics over into the Southern Ocean. But this

is not all; not only would the median-line of the trades be shifted

southwards, but the great equatorial currents of the globe would also

be shifted southwards.

Let us now consider how this would affect the Gulf-stream. The South

American continent is shaped somewhat in the form of a triangle, with

one of its angular corners, called Cape St. Roque, pointing eastwards.

The equatorial current of the Atlantic impinges against this corner;

but as the greater portion of the current lies a little to the north

of the corner, it flows westward into the Gulf of Mexico and forms the

Gulf-stream. A considerable portion of the water, however, strikes the

land to the south of the Cape and is deflected along the shores of

Brazil into the Southern Ocean, forming what is known as the Brazilian

current.

Now it is perfectly obvious that the shifting of the equatorial

current of the Atlantic only a few degrees to the south of its present

position—a thing which would certainly take place under the conditions

which we have been detailing—would turn the entire current into the

Brazilian branch, and instead of flowing chiefly into the Gulf of

Mexico as at present, it would all flow into the Southern Ocean, and

the Gulf-stream would consequently be stopped. The stoppage of the

Gulf-stream, combined with all those causes which we have just been

considering, would place Europe under glacial conditions; while, at the

same time, the temperature of the Southern Ocean would, in consequence

of the enormous quantity of warm water received, have its temperature

(already high from other causes) raised enormously.

_Deflection of the Gulf-stream during the Glacial Epoch indicated by

the Difference between the Clyde and Canadian Shell-beds._—That the

glaciation of north-western Europe resulted to a great extent from

the stoppage of the Gulf-stream may, I think, be inferred from a

circumstance pointed out by the Rev. Mr. Crosskey, several years ago,

in a paper read before the Glasgow Geological Society.[45] He showed

that the difference between the glacial shells of Canada and those

now existing in the Gulf of St. Lawrence is much less marked than the

difference between the glacial shells of the Clyde beds and those now

existing in the Firth. And from this he justly infers that the change

of climate in Canada since the glacial epoch has been far less complete

than in Scotland.

The return of the Gulf-stream has raised the mean annual temperature of

our island no less than 15° above the normal, while Canada, deprived of

its influence and exposed to a cold stream from polar regions, has been

kept nearly as much below the normal.

Let us compare the present temperature of the two countries. In making

our comparison we must, of course, compare places on the same latitude.

It will not do, for example, to compare Glasgow with Montreal or

Quebec, places on the latitude of the south of France and north of

Italy. It will be found that the difference of temperature between

the two countries is so enormous as to appear scarcely credible to

those who have not examined the matter. The temperatures have all been

taken from Professor Dove’s work on the “Distribution of Heat over the

Surface of the Globe,” and his Tables published in the Report of the

British Association for 1847.

The mean temperature of Scotland for January is about 38° F., while

in some parts of Labrador, on the same latitude, and all along the

central parts of North America lying to the north of Upper Canada,

it is actually 10°, and in many places 13° below zero. The January

temperature at the Cumberland House, which is situated on the latitude

of the centre of England, is more than 13° below zero. Here is a

difference of no less than 51°. The normal temperature for the month

of January in the latitude of Glasgow, according to Professor Dove, is

10°. Consequently, owing to the influence of the Gulf-stream, we are

28° warmer during that month than we would otherwise be, while vast

tracts of country in America are 23° colder than they should be.

The July temperature of Glasgow is 61°, while on the same latitude

in Labrador and places to the west it is only 49°. Glasgow during

that month is 3° above the normal temperature, while America, owing

to the influence of the cold polar stream, is 9° below it. The mean

annual temperature of Glasgow is nearly 50°, while in America, on the

same latitude, it is only 30°, and in many places as low as 23°. The

mean normal temperature for the whole year is 35°. Our mean annual

temperature is therefore 15° above the normal, and that of America from

5° to 12° below it. The American winters are excessively cold, owing

to the continental character of the climate, and the absence of any

benefit from the Gulf-stream, while the summers, which would otherwise

be warm, are, in the latitude of Glasgow, cooled down to a great extent

by the cold ice from Greenland; and the consequence is, that the mean

annual temperature is about 20° or 27° below that of ours. The mean

annual temperature of the Gulf of St. Lawrence is as low as that of

Lapland or Iceland. It is no wonder, then, that the shells which

flourished in Canada during the glacial epoch have not left the gulf

and the neighbouring seas.

We have good reason to believe that the climate of America during the

glacial epoch was even then somewhat more severe than that of Western

Europe, for the erratics of America extend as far south as latitude

40°, while on the old continent they are not found much beyond latitude

50°. This difference may have resulted from the fact that the western

side of a continent is always warmer than the eastern.

In order to determine whether the cold was as great in America during

the glacial epoch as in Western Europe, we must not compare the fossils

found in the glacial beds about Montreal, for example, with those found

in the Clyde beds, for Montreal lies much further to the south than the

Clyde. The Clyde beds must be compared with those of Labrador, while

the beds of Montreal must be compared with those of the south of France

and the north of Italy, if any are to be found there.

On the whole, it may be concluded that had the Gulf-stream not returned

to our shores at the close of the glacial epoch, and had its place

been supplied by a cold stream from the polar regions, similar to that

which washes the shores of North America, it is highly probable that

nearly every species found in our glacial beds would have had their

representatives flourishing in the British seas at the present day.

It is no doubt true that when we compare the places in which the

Canadian shell-beds referred to by Mr. Crosskey are situated with

places on the same latitude in Europe, the difference of climate

resulting from the influence of the Gulf-stream is not so great as

between Scotland and those places which we have been considering; but

still the difference is sufficiently great to account for why the

change of climate in Canada has been less complete than in Scotland.

And what holds true in regard to the currents of the Atlantic holds

also true, though perhaps not to the same extent, of the currents of

the Pacific.

_Nearness of the Sun in Perigee a Cause of the Accumulation of

Ice._—But there is still another cause which must be noticed:—A strong

under current of air _from_ the north implies an equally strong upper

current _to_ the north. Now if the effect of the under current would

be to impel the warm water at the equator to the south, the effect

of the upper current would be to carry the aqueous vapour formed at

the equator to the north; the upper current, on reaching the snow and

ice of temperate regions, would deposit its moisture in the form of

snow; so that, notwithstanding the great cold of the glacial epoch,

it is probable that the quantity of snow falling in the northern

regions would be enormous. This would be particularly the case during

summer, when the earth would be in the perihelion and the heat at the

equator great. The equator would be the furnace where evaporation would

take place, and the snow and ice of temperate regions would act as a

condenser.

Heat to produce _evaporation_ is just as essential to the accumulation

of snow and ice as cold to produce _condensation_. Now at Midsummer,

on the supposition of the eccentricity being at its superior limit,

the sun would be 8,641,870 miles nearer than at present during that

season. The effect would be that the intensity of the sun’s rays would

be one-fifth greater than now. That is to say, for every five rays

received by the ocean at present, six rays would be received then,

consequently the evaporation during summer would be excessive. But the

ice-covered land would condense the vapour into snow. It would, no

doubt, be during summer that the greatest snowfall would take place. In

fact, the nearness of the sun during that season was as essential to

the production of the glacial epoch as was his distance during winter.

The direct effect of eccentricity is to produce on one of the

hemispheres a long and cold winter. This alone would not lead to a

condition of things so severe as that which we know prevailed during

the glacial epoch. But the snow and ice thus produced would bring into

operation, as we have seen, a host of physical agencies whose combined

efforts would be quite sufficient to do this.

_A remarkable Circumstance regarding those Causes which lead to Secular

Changes of Climate._—There is one remarkable circumstance connected

with those physical causes which deserves special notice. They not only

all lead to one result, viz., an accumulation of snow and ice, but

they react on one another. It is quite a common thing in physics for

the effect to react on the cause. In electricity and magnetism, for

example, cause and effect in almost every case mutually act and react

upon each other. But it is usually, if not universally, the case that

the reaction of the effect tends to weaken the cause. The weakening

influences of this reaction tend to impose a limit on the efficiency

of the cause. But, strange to say, in regard to the physical causes

concerned in the bringing about of the glacial condition of climate,

cause and effect mutually reacted so as to strengthen each other. And

this circumstance had a great deal to do with the extraordinary results

produced.

We have seen that the accumulation of snow and ice on the ground

resulting from the long and cold winters tended to cool the air

and produce fogs which cut off the sun’s rays. The rays thus cut

off diminished the melting power of the sun, and so increased the

accumulation. As the snow and ice continued to accumulate, more and

more of the rays were cut off; and on the other hand, as the rays

continued to be cut off, the _rate_ of accumulation increased, because

the quantity of snow and ice melted became thus annually less and less.

Again, during the long and dreary winters of the glacial epoch the

earth would be radiating off its heat into space. Had the heat thus

lost simply gone to lower the temperature, the lowering of the

temperature would have tended to diminish the rate of loss; but the

necessary result of this was the formation of snow and ice rather than

the lowering of temperature.

And, again, the formation of snow and ice facilitated the rate at which

the earth lost its heat; and on the other hand, the more rapidly the

earth parted with its heat, the more rapidly were the snow and ice

formed.

Further, as the snow and ice accumulated on the one hemisphere, they

at the same time continued to diminish on the other. This tended to

increase the strength of the trade-winds on the cold hemisphere, and

to weaken those on the warm. The effect of this on ocean currents

would be to impel the warm water of the tropics more to the warm

hemisphere than to the cold. Suppose the northern hemisphere to be

the cold one, then as the snow and ice began gradually to accumulate

there, the ocean currents of that hemisphere would begin to decrease in

volume, while those on the southern, or warm, hemisphere, would _pari

passu_ increase. This withdrawal of heat from the northern hemisphere

would tend, of course, to lower the temperature of that hemisphere

and thus favour the accumulation of snow and ice. As the snow and ice

accumulated the ocean currents would decrease, and, on the other hand,

as the ocean currents diminished the snow and ice would accumulate,—the

two effects mutually strengthening each other.

The same must have held true in regard to aërial currents. The more

the polar and temperate regions became covered with snow and ice, the

stronger would become the trades and anti-trades of the hemisphere; and

the stronger those winds became, the greater would be the amount of

moisture transferred from the tropical regions by the anti-trades to

the temperate regions; and on the other hand, the more moisture those

winds brought to temperate regions, the greater would be the quantity

of snow produced.

The same process of mutual action and reaction would take place among

the agencies in operation on the warm hemisphere, only the result

produced would be diametrically opposite of that produced in the cold

hemisphere. On this warm hemisphere action and reaction would tend to

raise the mean temperature and diminish the quantity of snow and ice

existing in temperate and polar regions.

Had it been possible for each of those various physical agents which we

have been considering to produce its direct effects without influencing

the other agents or being influenced by them, its real efficiency in

bringing about either the glacial condition of climate or the warm

condition of climate would not have been so great.

The primary cause that set all those various physical agencies in

operation which brought about the glacial epoch, was a high state of

eccentricity of the earth’s orbit. When the eccentricity is at a

high value, snow and ice begin to accumulate, owing to the increasing

length and coldness of the winter on that hemisphere whose winter

solstice is approaching toward the aphelion. The accumulating snow

then begins to bring into operation all the various agencies which

we have been describing; and, as we have just seen, these, when once

in full operation, mutually aid one another. As the eccentricity

increases century by century, the temperate regions become more and

more covered with snow and ice, first by reason of the continued

increase in the coldness and length of the winters, and secondly,

and chiefly, owing to the continued increase in the potency of those

physical agents which have been called into operation. This glacial

state of things goes on at an increasing rate, and reaches a maximum

when the solstice-point arrives at the aphelion. After the solstice

passes the aphelion, a contrary process commences. The snow and ice

gradually begin to diminish on the cold hemisphere and to make their

appearance on the other hemisphere. The glaciated hemisphere turns, by

degrees, warmer and the warm hemisphere colder, and this continues to

go on for a period of ten or twelve thousand years, until the winter

solstice reaches the perihelion. By this time the conditions of the two

hemispheres have been reversed; the formerly glaciated hemisphere has

now become the warm one, and the warm hemisphere the glaciated. The

transference of the ice from the one hemisphere to the other continues

as long as the eccentricity remains at a high value. This will,

perhaps, be better understood from an inspection of the frontispiece.

_The Mean Temperature of the whole Earth should be greater in Aphelion

than in Perihelion._—When the eccentricity becomes reduced to about

its present value, its influence on climate is but little felt.

It is, however, probable that the present extension of ice on the

southern hemisphere may, to a considerable extent, be the result of

eccentricity. The difference in the climatic conditions of the two

hemispheres is just what should be according to theory:—(1) The mean

temperature of that hemisphere is less than that of the northern.

(2) The winters of the southern hemisphere are colder than those of

the northern. (3) The summers, though occurring in perihelion, are

also comparatively cold; this, as we have seen, is what ought to be

according to theory. (4) The mean temperature of the whole earth is

greater in June, when the earth is in aphelion, than in December, when

it is in perihelion. This, I venture to affirm, is also what ought to

follow according to theory, although this very fact has been adduced

as a proof that eccentricity has at present but little effect on the

climatic condition of our globe.

That the mean temperature of the whole earth would, during the

glacial epoch, be greater when the earth was in aphelion than

when in perihelion will, I think, be apparent from the following

considerations:—When the earth was in the perihelion, the sun would

be over the hemisphere nearly covered with snow and ice. The great

strength of the sun’s rays would in this case have little effect in

raising the temperature; it would be spent in melting the snow and

ice. But when the earth was in the aphelion, the sun would be over the

hemisphere comparatively free, or perhaps wholly free, from snow and

ice. Consequently, though the intensity of the sun’s rays would be less

than when the earth was in perihelion, still it ought to have produced

a higher temperature, because it would be chiefly employed in heating

the ground and not consumed in melting snow and ice.

_Professor Tyndall on the Glacial Epoch._—“So natural,” says Professor

Tyndall, “was the association of ice and cold, that even celebrated

men assumed that all that is needed to produce a great extension of

our glaciers is a diminution of the sun’s temperature. Had they gone

through the foregoing reflections and calculations, they would probably

have demanded _more_ heat instead of less for the production of a

glacial epoch. What they really needed were _condensers_ sufficiently

powerful to congeal the vapour generated by the heat of the sun.” (_The

Forms of Water_, p. 154. See also, to the same effect, _Heat Considered

as a Mode of Motion_, chap. vi.)

I do not know to whom Professor Tyndall here refers, but certainly his

remarks have no application to the theory under consideration, for

according to it, as we have just seen, the ice of the glacial epoch was

about as much due to the nearness of the sun in perigee as to his great

distance in apogee.

There is one theory, however, to which his remarks justly apply, viz.,

the theory that the great changes of climate during geological ages

resulted from the passage of our globe through different temperatures

of space. What Professor Tyndall says shows plainly that the glacial

epoch was not brought about by our earth passing through a cold part

of space. A general reduction of temperature over the whole globe

certainly would not produce a glacial epoch. Suppose the sun were

extinguished and our globe exposed to the temperature of stellar space

(−239° F.), this would certainly freeze the ocean solid from its

surface to its bottom, but it would not cover the land with ice.

Professor Tyndall’s conclusions are, of course, equally conclusive

against Professor Balfour Stewart’s theory, that the glacial epoch may

have resulted from a general diminution in the intensity of the sun’s

heat.

Nevertheless it would be in direct opposition to the well-established

facts of geology to assume that the ice periods of the glacial epoch

were warm periods. We are as certain from palæontological evidence

that the cold was then much greater than now, as we are from physical

evidence that the accumulation of ice was greater than now. Our glacial

shell-beds and remains of the mammoth, the reindeer, and musk-ox, tell

of cold as truly as the markings on the rocks do of ice.

_Objection from the Present Condition of the Planet Mars._—It has been

urged as an objection by Professor Charles Martins[46] and others,

that if a high state of eccentricity could produce a glacial epoch,

the planet Mars ought to be at present under a glacial condition. The

eccentricity of its orbit amounts to 0·09322, and one of its southern

winter solstices is, according to Dr. Oudemans, of Batavia,[47] within

17° 41′ 8″ of aphelion. Consequently, it is supposed that one of the

hemispheres should be in a glacial state and the other free from snow

and ice. But it is believed that the snow accumulates around each pole

during its winter and disappears to a great extent during its summer.

There would be force in this objection were it maintained that

eccentricity alone can produce a glacial condition of climate, but

such is not the case, and there is no good ground for concluding that

those physical agencies which led to the glacial epoch of our globe

exist in the planet Mars. It is perfectly certain that either water

must be different in constitution in that planet from what it is in our

earth, or else its atmospheric envelope must be totally different from

ours. For it is evident from what has been stated in Chapter II., that

were our globe to be removed to the distance of Mars from the sun, the

lowering of the temperature resulting from the decrease in the sun’s

heat would not only destroy every living thing, but would convert the

ocean into solid ice.

But it must be observed that the eccentricity of Mars’ orbit is at

present far from its superior limit of 0·14224, and it may so happen in

the economy of nature that when it approaches to that limit a glacial

condition of things may supervene.

The truth is, however, that very little seems to be known with

certainty regarding the climatic condition of Mars. This is obvious

from the fact that some astronomers believe that the planet possesses

a dense atmosphere which protects it from cold; while others maintain

that its atmosphere is so exceedingly thin that its mean temperature is

below the freezing-point. Some assert that the climatic condition of

Mars resembles very much that of our earth, while others affirm that

its seas are actually frozen solid to the bottom, and the poles covered

with ice thirty or forty miles in thickness. For reasons which will be

explained in the Appendix, Mars, notwithstanding its greater distance

from the sun, may enjoy a climate as warm as that of our earth.

CHAPTER V.

REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE

NORTHERN.

Adhémar’s Explanation.—Adhémar’s Theory founded upon a physical

Mistake in regard to Radiation.—Professor J. D. Forbes on

Underground Temperature.—Generally accepted Explanation.—Low

Temperature of Southern Hemisphere attributed to

Preponderance of Sea.—Heat transferred from Southern to

Northern Hemisphere by Ocean-current the true Explanation.—A

large Portion of the Heat of the Gulf-stream derived from the

Southern Hemisphere.

_Adhémar’s Explanation._—It has long been known that on the southern

hemisphere the temperature is lower and the accumulation of ice greater

than on the northern. This difference has usually been attributed to

the great preponderance of sea on the southern hemisphere. M. Adhémar,

on the other hand, attempts to explain this difference by referring it

to the difference in the amount of heat lost by the two hemispheres

in consequence of the difference of seven days in the length of their

respective winters. As the northern winter is shorter than the summer,

he concludes that there is an accumulation of heat on that hemisphere,

while, on the other hand, the southern winter being longer than the

summer, there is therefore a loss of heat on the southern hemisphere.

“The south pole,” he says, “loses in one year more heat than it

receives, because the total duration of its night surpasses that of

its day by 168 hours; and the contrary takes place for the north pole.

If, for example, we take for unity the mean quantity of heat which the

sun sends off in one hour, the heat accumulated at the end of the year

at the north pole will be expressed by 168, while the heat lost by the

south pole will be equal to 168 times what the radiation lessens it by

in one hour, so that at the end of the year the difference in the heat

of the two hemispheres will be represented by 336 times what the earth

receives from the sun or loses in an hour by radiation.”[48]

Adhémar supposes that about 10,000 years hence, when our northern

winter will occur in aphelion and the southern in perihelion, the

climatic conditions of the two hemispheres will be reversed; the

ice will melt at the south pole, and the northern hemisphere will

become enveloped in one continuous mass of ice, leagues in thickness,

extending down to temperate regions.

This theory seems to be based upon an erroneous interpretation of a

principle, first pointed out, so far as I am aware, by Humboldt in

his memoir “On Isothermal Lines and Distribution of Heat over the

Globe.”[49] This principle may be stated as follows:—

Although the total quantity of heat received by the earth from the

sun in one revolution is inversely proportional to the minor axis of

the orbit, yet this amount, as was proved by D’Alembert, is equally

distributed between the two hemispheres, whatever the eccentricity may

be. Whatever extra heat the southern hemisphere may at present receive

from the sun daily during its summer months owing to greater proximity

to the sun, is exactly compensated by a corresponding loss arising from

the shortness of the season; and, on the other hand, whatever daily

deficiency of heat we in the northern hemisphere may at present have

during our summer half-year, in consequence of the earth’s distance

from the sun, is also exactly compensated by a corresponding length of

season.

But the surface temperature of our globe depends as much upon the

amount of heat radiated into space as upon the amount derived from the

sun, and it has been thought by some that this compensating principle

holds true only in regard to the latter. In the case of the heat

lost by radiation the reverse is supposed to take place. The southern

hemisphere, it is asserted, has not only a colder winter than the

northern in consequence of the sun’s greater distance, but it has also

a longer winter; and the extra loss of heat from radiation during

winter is not compensated by its nearness to the sun during summer, for

it gains no additional heat from this proximity. And in the same way it

is argued that as our winter in the northern hemisphere, owing to the

less distance of the sun, is not only warmer than that of the southern

hemisphere, but is also at the same time shorter, so our hemisphere

is not cooled to such an extent as the southern. And thus the mean

temperature of the winter half-year, as well as the intensity of the

sun’s heat, is affected by a change in the sun’s distance.

Although I always regarded this cause of Humboldt’s to be utterly

inadequate to produce such effects as those attributed to it by

Adhémar, still, in my earlier papers[50] I stated it to be a _vera

causa_ which ought to produce some sensible effect on climate. But

shortly afterwards on a more careful consideration of the whole

subject, I was led to suspect that the circumstance in question can,

according to theory, produce little or no effect on the climatic

condition of our globe.

As there appears to be a considerable amount of misapprehension in

reference to this point, which forms the basis of Adhémar’s theory, I

may here give it a brief consideration.[51]

The rate at which the earth radiates into space the heat received

from the sun depends upon the temperature of its surface; and the

temperature of its surface (other things being equal) depends upon

the rate at which the heat is received. The greater the rate at which

the earth receives heat from the sun, the greater will therefore be

the rate at which it will lose that heat by radiation. Now the total

quantity of heat received during winter by the southern hemisphere is

exactly equal to that received during winter by the northern. But as

the southern winter is longer than the northern, the rate at which the

heat is received, and consequently the rate of radiation, during that

season must be less on the southern hemisphere than on the northern.

Thus the southern hemisphere loses heat during a longer period than the

northern, and therefore the less rate of radiation (were it not for a

circumstance presently to be noticed) would wholly compensate for the

longer period, and the total quantity of heat lost during winter would

be the same on both hemispheres. The southern summer is shorter than

the northern, but the heat is more intense, and the surface of the

ground kept at a higher temperature; consequently the rate of radiation

into space is greater.

When the rate at which a body receives heat is increased, the

temperature of the body rises till the rate of radiation equals the

rate of absorption, after which equilibrium is restored; and when the

rate of absorption is diminished, the temperature falls till the rate

of radiation equals that of absorption.

But notwithstanding all this, owing to the slow conductivity of the

ground for heat, more heat will pass into it during the longer summer

of aphelion than during the shorter one of perihelion; for the amount

of heat which passes into the ground depends on the length of time

during which the earth is receiving heat, as well as upon the amount

received. In like manner, more heat will pass out of the ground

during the longer winter in aphelion than during the shorter one in

perihelion. Suppose the length of the days on the one hemisphere (say

the northern) to be 23 hours, and the length of the nights, say one

hour; while on the other hemisphere the days are one hour and the

nights 23 hours. Suppose also that the quantity of heat received from

the sun by the southern hemisphere during the day of one hour to be

equal to that received by the northern hemisphere during the day of

23 hours. It is evident that although the surface of the ground on

the southern hemisphere would receive as much heat from the sun during

the short day of one hour as the surface of the northern hemisphere

during the long day of 23 hours, yet, owing to the slow conductivity

of the ground for heat, the amount absorbed would not be nearly so

much on the southern hemisphere as on the northern. The temperature

of the surface during the day, it is true, would be far higher on the

southern hemisphere than on the northern, and consequently the rate

at which the heat would pass into the ground would be greater on that

hemisphere than on the northern; but, notwithstanding the greater rate

of absorption resulting from the high temperature of the surface, it

would not compensate for the shortness of the day. On the other hand,

the surface of the ground on the southern hemisphere would be colder

during the long night of 23 hours than it would be on the northern

during the short night of only one hour; and the low temperature of the

ground would tend to lessen the rate of radiation into space. But the

decrease in the rate of radiation would not compensate fully for the

great length of the night. The general and combined result of all those

causes would be that a slight accumulation of heat would take place on

the northern hemisphere and a slight loss on the southern. But this

loss of heat on the one hemisphere and gain on the other would not go

on accumulating at a uniform rate year by year, as Adhémar supposes.

Of course we are at present simply considering the earth as an absorber

and radiator of heat, without taking into account the effects of

distribution of sea and land and other modifying causes, and are

assuming that everything is the same in both hemispheres, with the

exception that the winter of the one hemisphere is longer than that of

the other.

What, then, is the amount of heat stored up by the one hemisphere and

lost by the other? Is it such an amount as to sensibly affect climate?

The experiments and observations which have been made on underground

temperature afford us a means of making at least a rough estimate of

the amount. And from these it will be seen that the influence of an

excess of seven or eight days in the length of the southern winter over

the northern could hardly produce an effect that would be sensible.

Observations were made at Edinburgh by Professor J. D. Forbes on

three different substances; viz., sandstone, sand, and trap-rock. By

calculation, we find from the data afforded by those observations that

the total quantity of heat accumulated in the ground during the summer

above the mean temperature was as follows:—In the sandstone-rock, a

quantity sufficient to raise the temperature of the rock 1° C. to a

depth of 85 feet 6 inches; in the sand a quantity sufficient to raise

the temperature 1° C. to a depth of 72 feet 6 inches; and in the

trap-rock a quantity only sufficient to raise the temperature 1° C. to

a depth of 61 feet 6 inches.

Taking the specific heat of the sandstone per unit volume, as

determined by Regnault, at ·4623, and that of sand at ·3006, and

trap at ·5283, and reducing all the results to one standard, viz.,

that of water, we find that the quantity of heat stored up in the

sandstone would, if applied to water, raise its temperature 1° C. to

a depth of 39 feet 6 inches; that stored up in the sand would raise

the temperature of the water 1° C. to a depth of 21 feet 8 inches, and

that stored up in the trap would raise the water 1° C. to the depth

of 32 feet 6 inches. We may take the mean of these three results as

representing pretty accurately the quantity stored up in the general

surface of the country. This would be equal to 31 feet 3 inches depth

of water raised 1° C. The quantity of heat lost by radiation during

winter below the mean was found to be about equal to that stored up

during summer.

The total quantity of heat per square foot of surface received by the

equator from sunrise till sunset at the time of the equinoxes, allowing

22 per cent. for the amount cut off in passing through the atmosphere,

is 1,780,474 foot-pounds. In the latitude of Edinburgh about 938,460

foot-pounds per square foot of surface is received, assuming that not

more than 22 per cent. is cut off by the atmosphere. At this rate a

quantity of heat would be received from the sun in two days ten hours

(say, three days) sufficient to raise the temperature of the water 1°

C. to the required depth of 31 feet 3 inches. Consequently the total

quantity of heat stored up during summer in the latitude of Edinburgh

is only equal to what we receive from the sun during three days at the

time of the equinoxes. Three days’ sunshine during the middle of March

or September, if applied to raise the temperature of the ground, would

restore all the heat lost during the entire winter; and another three

days’ sunshine would confer on the ground as much heat as is stored

up during the entire summer. But it must be observed that the total

duration of sunshine in winter is to that of summer in the latitude of

Edinburgh only about as 4 to 7. Here is a difference of two months.

But this is not all; the quantity of heat received during winter is

scarcely one-third of that received during summer; yet, notwithstanding

this enormous difference between summer and winter, the ground during

winter loses only about six days’ sun-heat below the maximum amount

possessed by it in summer.

But if what has already been stated is correct, this loss of heat

sustained by the earth during winter is not chiefly owing to radiation

during the longer absence of the sun, but to the decrease in the

quantity of heat received in consequence of his longer absence combined

with the obliquity of his rays during that season. Now in the case

of the two hemispheres, although the southern winter is longer than

the northern, yet the quantity of heat received by each is the same.

But supposing it held true, which it does not, that the loss of

heat sustained by the earth in winter is as much owing to radiation

resulting from the excess in the length of the winter nights over those

of the summer as to the deficiency of heat received in winter from that

received in summer, three days’ heat would then in this case be the

amount lost by radiation in consequence of this excess in the length of

the winter nights. The total length of the winter nights to those of

the summer is, as we have seen, about as 7 to 4. This is a difference

of nearly 1200 hours. But the excess of the south polar winter over the

north amounts to only about 184 hours. Now if 1200 hours give a loss of

three days’ sun-heat, 184 hours will give a loss of scarcely 5½ hours.

It is no doubt true that the two cases are not exactly analogous; but

it is obvious that any error which can possibly arise from regarding

them as such cannot materially alter the conclusion to which we have

arrived. Supposing the effect were double, or even quadruple, what

we have concluded it to be, still it would not amount to a loss of

two days’ heat, which could certainly have little or no influence on

climate.

But even assuming all the preceding reasoning to be incorrect, and that

the southern hemisphere, in consequence of its longer winter, loses

heat to the extravagant extent of 168 hours, supposed by Adhémar, still

this could not materially affect climate. The climate is influenced

by the mere _temperature_ of the _surface_ of the ground, and not by

the quantity of heat or cold that may be stored up under the surface.

The climate is determined, so far as the ground is concerned, by

the temperature of the surface, and is wholly independent of the

temperature which may exist under the surface. Underground temperature

can only affect climate through the surface. If the surface could,

for example, be kept covered with perpetual snow, we should have a

cold and sterile climate, although the temperature of the ground under

the snow was actually at the boiling-point. Let the ground to a depth

of, say 40 or 50 feet, be deprived of an amount of heat equal to that

received from the sun in 168 hours. This could produce little or no

sensible effect on climate; for, owing to the slow conductivity of the

ground for heat, this loss would not sensibly affect the temperature

of the surface, as it would take several months for the sun’s heat

to penetrate to that depth and restore the lost heat. The cold, if I

may be allowed to use the expression, would come so slowly out to the

surface that its effect in lowering the temperature of the surface

would scarcely be sensible. And, again, if we suppose the 168 hours’

heat to be lost by the mere surface of the ground, the effect would

certainly be sensible, but it would only be so for a few days. We

might in this case have a week’s frozen soil, but that would be all.

Before the air had time to become very sensibly affected by the low

temperature of the surface the frozen soil would be thawed.

The storing up of heat or cold in the ground has in reality very little

to do with climate. Some physicists explain, for example, why the month

of July is warmer than June by referring it to the fact that by the

month of July the ground has become possessed of a larger accumulation

of heat than it possessed in June. This explanation is evidently

erroneous. The ground in July certainly possesses a greater store of

heat than it did in June; but this is not the reason why the former

month is hotter than the latter. July is hotter than June because the

_air_ (not the _ground_) has become possessed of a larger store of

heat than it had in June. Now the air is warmer in July than in June

because, receiving little increase of temperature from the direct rays

of the sun, it is heated chiefly by radiation from the earth and by

contact with its warm surface. Consequently, although the sun’s heat

is greater in June than it is in July, it is near the middle of July

before the air becomes possessed of its maximum store of heat. We

therefore say that July is hotter than June because the air is hotter,

and consequently the temperature in the shade is greater in the former

month than in the latter.

It is therefore, I presume, quite apparent that Adhémar’s theory fails

to explain why the southern hemisphere is colder than the northern.

_The generally accepted Explanation._—The difference in the mean

temperature of the two hemispheres is usually attributed to the

proportion of sea to land in the southern hemisphere and of land to

sea in the northern hemisphere. This, no doubt, will account for the

greater _annual range_ of temperature on the northern hemisphere,

but it seems to me that it will not account for the excess of _mean_

temperature possessed by that hemisphere over the southern.

The general influence of land on climate is to exaggerate the

variation of temperature due to the seasons. On continents the summers

are hotter and the winters colder than on the ocean. The days are

also hotter and the nights colder on land than on sea. This is a

result which follows from the mere physical properties of land and

water, independently of currents, whether of ocean or of air. But it

nevertheless follows, according to theory (and this is a point which

has been overlooked), that the mean annual temperature of the ocean

ought to be greater than that of the land in equatorial regions as

well as in temperate and polar regions. This will appear obvious for

the following reasons:—(1) The ground stores up heat only by the slow

process of conduction, whereas water, by the mobility of its particles

and its transparency for heat-rays, especially those from the sun,

becomes heated to a considerable depth rapidly. The quantity of heat

stored up in the ground is thus comparatively small, while the quantity

stored up in the ocean is great. (2) The air is probably heated more

rapidly by contact with the ground than with the ocean; but, on the

other hand, it is heated far more rapidly by radiation from the ocean

than from the land. The aqueous vapour of the air is to a great extent

diathermanous to radiation from the ground, while it absorbs the

rays from water and thus becomes heated. (3) The air radiates back a

considerable portion of its heat, and the ocean absorbs this radiation

from the air more readily than the ground does. The ocean will not

reflect the heat from the aqueous vapour of the air, but absorbs it,

while the ground does the opposite. Radiation from the air, therefore,

tends more readily to heat the ocean than it does the land. (4) The

aqueous vapour of the air acts as a screen to prevent the loss by

radiation from water, while it allows radiation from the ground to pass

more freely into space; the atmosphere over the ocean consequently

throws back a greater amount of heat than is thrown back by the

atmosphere over the land. The sea in this case has a much greater

difficulty than the land has in getting quit of the heat received from

the sun; in other words, the land tends to lose its heat more rapidly

than the sea. The consequence of all these circumstances is that the

ocean must stand at a higher mean temperature than the land. A state of

equilibrium is never gained until the rate at which a body is receiving

heat is equal to the rate at which it is losing it; but as equal

surfaces of sea and land receive from the sun the same amount of heat,

it therefore follows that, in order that the sea may get quit of its

heat as rapidly as the land, it _must stand at a higher temperature_

than the land. The temperature of the sea must continue to rise till

the amount of heat thrown off into space equals that received from the

sun; when this point is reached, equilibrium is established and the

temperature remains stationary. But, owing to the greater difficulty

that the sea has in getting rid of its heat, the mean temperature

of equilibrium of the ocean must be higher than that of the land;

consequently the mean temperature of the ocean, and also of the air

immediately over it, in tropical regions should be higher than the mean

temperature of the land and the air over it.

The greater portion of the southern hemisphere, however, is occupied by

water, and why then, it may be asked, is this water hemisphere colder

than the land hemisphere? Ought it not also to follow that the sea in

inter-tropical regions should be warmer than the land under the same

parallels; yet, as we know, the reverse is actually found to be the

case. How then is all this to be explained, if the foregoing reasoning

be correct? We find when we examine Professor Dove’s charts of mean

annual temperature, that the ocean in inter-tropical regions has a

mean annual temperature below the normal, and the land a mean annual

temperature above the normal. Both in the Pacific and in the Atlantic

the mean temperature sinks to 2°·3 below the normal, while on the

land it rises 4°·6 above the normal. The explanation in this case is

obviously this: the temperature of the ocean in inter-tropical regions,

as we have already seen, is kept much lower than it would otherwise be

by the enormous amount of _heat_ that is being constantly carried away

from those regions into temperate and polar regions, and of _cold_ that

is being constantly carried from temperate and polar regions to the

tropical regions by means of ocean-currents. The same principle which

explains why the sea in inter-tropical regions has a lower mean annual

temperature than the land, explains also why the southern hemisphere

has a lower mean annual temperature than the northern. The temperature

of the southern hemisphere is lowered by the transference of heat by

means of ocean-currents.

_Heat transferred from the Southern to the Northern Hemisphere by

Ocean-currents the true Explanation._—The great ocean-currents of

the globe take their rise in three immense streams from the Southern

Ocean, which, on reaching the tropical regions, become deflected in

a westerly direction and flow along the southern side of the equator

for thousands of miles. Perhaps more than one half of this mass of

moving water returns into the Southern Ocean without ever crossing the

equator, but the quantity which crosses over to the northern hemisphere

is enormous. This constant flow of water from the southern hemisphere

to the northern in the form of surface currents must be compensated by

_under currents_ of equal magnitude from the northern hemisphere to the

southern. The currents, however, which cross the equator are far higher

in temperature than their compensating under currents; consequently

there is a constant transference of heat from the southern hemisphere

to the northern. Any currents taking their rise in the northern

hemisphere and flowing across into the southern are comparatively

trifling, and the amount of heat transferred by them is also trifling.

There are one or two currents of considerable size, such as the

Brazilian branch of the great equatorial current of the Atlantic, and

a part of the South Equatorial Drift-current of the Pacific, which

cross the equator from north to south; but these cannot be regarded as

northern currents; they are simply southern currents deflected back

after crossing over to the northern hemisphere. The heat which these

currents possess is chiefly obtained on the southern hemisphere before

crossing over to the northern; and although the northern hemisphere may

not gain much heat by means of them, it, on the other hand, does not

lose much, for the heat which they give out in their progress along the

southern hemisphere does not belong to the northern hemisphere.

But, after making the fullest allowance for the amount of heat carried

across the equator from the northern hemisphere to the southern, we

shall find, if we compare the mean temperature of the currents from

south to north with that of the great compensating under currents and

the one or two small surface currents, that the former is very much

higher than the latter. The mean temperature of the water crossing the

equator from south to north is probably not under 65°, that of the

under currents is probably not over 39°. But to the under currents

we must add the surface currents from north to south; and assuming

that this will raise the mean temperature of the entire mass of water

flowing south to, say, 45°, we have still a difference of 20° between

the temperature of the masses flowing north and south. Each cubic

foot of water which crosses the equator will in this case transfer

about 965,000 foot-pounds of heat from the southern hemisphere to the

northern. If we had any means of ascertaining the volume of those great

currents crossing the equator, we should then be able to make a rough

estimate of the total amount of heat transferred from the southern

hemisphere to the northern; but as yet no accurate estimate has been

made on this point. Let us assume, what is probably below the truth,

that the total amount of water crossing the equator is at least double

that of the Gulf-stream as it passes through the Straits of Florida,

which amount we have already found to be equal to 66,908,160,000,000

cubic feet daily. Taking the quantity of heat conveyed by each cubic

foot of water of the Gulf-stream as 1,158,000 foot-pounds, it is

found, as we have seen, that an amount of heat is conveyed by this

current equal to all the heat that falls within 32 miles on each

side of the equator. Then, if each cubic foot of water crossing the

equator transfers 965,000 foot-pounds, and the quantity of water be

double that of the Gulf-stream, it follows that the amount of heat

transferred from the southern hemisphere to the northern is equal to

all the heat falling within 52 miles on each side of the equator, or

equal to all the heat falling on the southern hemisphere within 104

miles of the equator. This quantity taken from the southern hemisphere

and added to the northern will therefore make a difference in the

amount of heat possessed by the two hemispheres equal to all the heat

which falls on the southern hemisphere within somewhat more than 208

miles of the equator.

_A large Portion of the Heat of the Gulf-stream derived from the

Southern Hemisphere._—It can be proved that a very large portion of the

heat conveyed by the Gulf-stream comes from the southern hemisphere.

The proof is as follows:—

If all the heat came from the northern hemisphere, it could only come

from that portion of the Atlantic, Caribbean Sea, and Gulf of Mexico

which lies to the north of the equator. The entire area of these seas,

extending to the Tropic of Cancer, is about 7,700,000 square miles.

But this area is not sufficient to supply the current passing through

the “Narrows” with the necessary heat. Were the heat which passes

through the Straits of Florida derived exclusively from this area, the

following table would then represent the relative quantity per unit

surface possessed by the Atlantic in the three zones, assuming that one

half of the heat of the Gulf-stream passes into the arctic regions and

the other half remains to warm the temperate regions[52]:—

From the equator to the Tropic of Cancer 773

From the Tropic of Cancer to the Arctic Circle 848

From the Arctic Circle to the North Pole 610

These figures show that the Atlantic, from the equator to the Tropic

of Cancer, would be as cold as from the Tropic of Cancer to the North

Pole, were it not that a large proportion of the heat possessed by the

Gulf-stream is derived from the southern hemisphere.

CHAPTER VI.

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC

CIRCULATION.—LIEUT. MAURY’S THEORY.

Introduction.—Ocean-currents, according to Maury, due to

Difference of Specific Gravity.—Difference of Specific

Gravity resulting from Difference of Temperature.—Difference

of Specific Gravity resulting from Difference of

Saltness.—Maury’s two Causes neutralize each other.—How,

according to him, Difference in Saltness acts as a Cause.

_Introduction._—Few subjects have excited more interest and attention

than the cause of ocean circulation; and yet few are in a more

imperfect and unsatisfactory condition, nor is there any question

regarding which a greater diversity of opinion has prevailed. Our

incomplete acquaintance with the facts relating to the currents of the

ocean and the modes of circulation actually in operation, is no doubt

one reason for this state of things. But doubtless the principal cause

of such diversity of opinion lies in the fact that the question is one

which properly belongs to the domain of physics and mechanics, while

as yet no physicist of note (if we except Dr. Colding, of Copenhagen)

has given, as far as I know, any special attention to the subject. It

is true that in works of meteorology and physical geography reference

is continually made to such eminent physicists as Herschel, Pouillet,

Buff, and others; but when we turn to the writings of these authors we

find merely a few remarks expressive of their opinions on the subject,

and no special discussion or investigation of the matter, nor anything

which could warrant us in concluding that such investigations have ever

been made. At present the question cannot be decided by a reference to

authorities.

The various theories on the subject may be classed under two divisions;

the first of these attributes the motion of the water to the _impulse

of the wind_, and the second to the _force of gravity_ resulting from

difference of density. But even amongst those who adopt the former

theory, it is generally held that the winds are not the sole cause,

but that, to a certain extent at least, difference of specific gravity

contributes to produce motion of the waters. This is a very natural

conclusion; and in the present state of physical geography on this

subject one can hardly be expected to hold any other view.

The supporters of the latter theory may be subdivided into two

classes. The first of these (of which Maury may be regarded as the

representative) attributes the Gulf-stream, and other sensible currents

of the ocean, to difference of specific gravity. The other class (at

present the more popular of the two, and of which Dr. Carpenter may be

considered the representative) denies altogether that such currents can

be produced by difference of specific gravity,[53] and affirms that

there is a general movement of the upper portion of the ocean from the

equator to the poles, and a counter-movement of the under portion from

the poles to the equator. This movement is attributed to difference of

specific gravity between equatorial and polar water, resulting from

difference of temperature.

The widespread popularity of the gravitation theory is no doubt, to a

great extent, owing to the very great prominence given to it by Lieut.

Maury in his interesting and popular work, “The Physical Geography of

the Sea.” Another cause which must have favoured the reception of this

theory is the ease with which it is perceived how, according to it,

circulation of the waters of the ocean is supposed to follow. One has

no difficulty, for example, in perceiving that if the inter-tropical

waters of the ocean are expanded by heat, and the waters around the

poles contracted by cold, the surface of the ocean will stand at a

higher level at the equator than at the poles. Equilibrium being

thus disturbed, the water at the equator will tend to flow towards

the poles as a surface current, and the water at the poles towards

the equator as an under current. This, at first sight, looks well,

especially to those who take but a superficial view of the matter.

We shall examine this theory at some length, for two reasons: 1,

because it lies at the root of a great deal of the confusion and

misconception which have prevailed in regard to the whole subject of

ocean-currents: 2, because, if the theory is correct, it militates

strongly against the physical theory of secular changes of climate

advanced in this volume. We have already seen (Chapter IV.) that

when the eccentricity of the earth’s orbit reaches a high value, a

combination of physical circumstances tends to lower the temperature of

the hemisphere which has its winter solstice in aphelion, and to raise

the temperature of the opposite hemisphere, whose winter solstice will,

of course, be in perihelion. The direct result of this state of things,

as was shown, is to strengthen the force of the trade-winds on the

cold hemisphere, and to weaken their strength on the warm hemisphere:

and this, in turn, we also saw, tends to impel the warm water of

the inter-tropical region on to the warm hemisphere, and to prevent

it, in a very large degree, from passing into the cold hemisphere.

This deflection of the ocean-currents tends to an enormous extent to

increase the difference of temperature previously existing between the

two hemispheres. In other words, the warm and equable condition of the

one hemisphere, and the cold and glacial condition of the other, are,

to a great extent, due to this deflection of ocean-currents. But if

the theory be correct which attributes the motion of ocean-currents to

a difference in density between the sea in inter-tropical and polar

regions, then it follows that these currents (other things being

equal) ought to be stronger on the cold hemisphere than on the warm,

because there is a greater difference of temperature and, consequently,

a greater difference of density, between the polar seas of the cold

hemisphere and the equatorial seas, than between the polar seas of the

warm hemisphere and the equatorial seas. And this being the case,

notwithstanding the influence of the trade-winds of the cold hemisphere

blowing over upon the warm, the currents will, in all probability,

be stronger on the cold hemisphere than on the warm. In other words,

the influence of the powerful trade-winds of the cold hemisphere to

transfer the warm water of the equator to the warm hemisphere will

probably be more than counterbalanced by the tendency of the warm

and buoyant waters of the equator to flow towards the dense and cold

waters around the pole of the cold hemisphere. But if ocean-currents

are due not to difference in specific gravity, but to the influence of

the winds, then it is evident that the waters at the equator will be

impelled, not into the cold hemisphere, but into the warm.

For this reason I have been the more anxious to prove that

inter-tropical heat is conveyed to temperate and polar regions by

ocean-currents, and not by means of any general movement of the ocean

resulting from difference of gravity. I shall therefore on this account

enter more fully into this part of the subject than I otherwise would

have done. Irrespective of all this, however, the important nature of

the whole question, and the very general interest it excites, warrant a

full consideration of the subject.

I shall consider first that form of the gravitation theory advocated

by Maury in his work on the “Physical Geography of the Sea,” which

attributes the motion of the Gulf-stream and other sensible currents

of the ocean to differences of specific gravity. One reason which has

induced me to select Maury’s work is, that it not only contains a much

fuller discussion on the cause of the motion of ocean-currents than is

to be found anywhere else, but also that it has probably passed through

a greater number of editions than any other book of a scientific

character in the English language in the same length of time.

_Examination of Lieut. Maury’s Gravitation Theory._—Although Lieut.

Maury has expounded his views on the cause of ocean-currents at

great length in the various editions of his work, yet it is somewhat

difficult to discover what they really are. This arises chiefly

from the generally confused and sometimes contradictory nature of

his hydrodynamical conceptions. After a repeated perusal of several

editions of his book, the following, I trust, will be found to be a

pretty accurate representation of his theory:—

_Ocean-currents, according to Maury, due to Difference of Specific

Gravity._—Although Maury alludes to a number of causes which, he

thinks, tend to produce currents, yet he deems their influence so

small that, practically, all currents may be referred to difference of

specific gravity.

“If we except,” he says, “the tides, and the partial currents of the

sea, such as those that may be created by the wind, we may lay it down

as a rule that all the currents of the ocean owe their origin to the

differences of specific gravity between sea-water at one place and

sea-water at another; for wherever there is such a difference, whether

it be owing to difference of temperature or to difference of saltness,

&c., it is a difference that disturbs equilibrium, and currents are the

consequence” (§ 467)[54]. To the same effect see §§ 896, 37, 512, 520,

and 537.

Notwithstanding the fact that he is continually referring to difference

of specific gravity as the great cause of currents, it is difficult to

understand in what way he conceives this difference to act as a cause.

Difference of specific gravity between the waters of the ocean at one

place and another can give rise to currents only through the influence

of the earth’s gravity. All currents resulting from difference of

specific gravity can be ultimately resolved into the general principle

that the molecules that are specifically heavier _descend_ and displace

those that are specifically lighter. If, for example, the ocean at the

equator be expanded by heat or by any other cause, it will be forced by

the denser waters in temperate and polar regions to rise so that its

surface shall stand at a higher level than the surface of the ocean in

these regions. The surface of the ocean will become an inclined plane,

sloping from the equator to the poles. Hydro-statically, the ocean,

considered as a mass, will then be in a state of equilibrium; but the

individual molecules will not be in equilibrium. The molecules at the

surface in this case may be regarded as lying on an inclined plane

sloping from the equator down to the poles, and as these molecules

are at liberty to move they will not remain at rest, but will descend

the incline towards the poles. When the waters at the equator are

expanded, or the waters at the poles contracted, gravitation makes, as

it were, a twofold effort to restore equilibrium. It in the first place

sinks the waters at the poles, and raises the waters at the equator,

in order that the two masses may balance each other; but this very

effort of gravitation to restore equilibrium to the mass destroys the

equilibrium of the molecules by disturbing the level of the ocean. It

then, in the second place, endeavours to restore equilibrium to the

molecules by pulling the lighter surface water at the equator down the

incline towards the poles. This tends not only to restore the level

of the ocean, but to bring the lighter water to occupy the surface

and the denser water the bottom of the ocean; and when this is done,

complete equilibrium is restored, both to the mass of the ocean and

to its individual molecules, and all further motion ceases. But if

heat be constantly applied to the waters of the equatorial regions,

and cold to those of the polar regions, and a permanent disturbance of

equilibrium maintained, then the continual effort of gravitation to

restore equilibrium will give rise to a constant current. In this case,

the heat and the cold (the agents which disturb the equilibrium of the

ocean) may be regarded as causes of the current, inasmuch as without

them the current would not exist; but the real efficient cause, that

which impels the water forward, is the force of gravity. But the force

of gravity, as has already been noticed, cannot produce motion (perform

work) unless the thing acted upon _descend_. Descent is implied in

the very conception of a current produced by difference of specific

gravity.

But Maury speaks as if difference of specific gravity could give rise

to a current without any descent.

“It is not necessary,” he says, “to associate with oceanic currents

the idea that they must of necessity, as on land, run from a higher to

a lower level. So far from this being the case, some currents of the

sea actually run up hill, while others run on a level. The Gulf-stream

is of the first class” (§ 403). “The top of the Gulf-stream runs on a

level with the ocean; therefore we know it is not a descending current”

(§ 18). And in § 9 he says that between the Straits of Florida and

Cape Hatteras the waters of the Gulf-stream “are actually forced up an

inclined plane, whose submarine ascent is not less than 10 inches to

the mile.” To the same effect see §§ 25, 59.

It is perfectly true that “it is not necessary to associate with

ocean-currents the idea that they must of necessity, as on land,

run from a higher to a lower level.” But the reason of this is that

ocean-currents do not, like the currents on land, owe their motion to

the force of gravitation. If ocean-currents result from difference of

specific gravity between the waters in tropical and polar regions,

as Maury maintains, then it is necessary to assume that they are

descending currents. Whatever be the cause which may give rise to a

difference of specific gravity, the motion which results from this

difference is due wholly to the force of gravity; but gravity can

produce no motion unless the water _descend_.

This fact must be particularly borne in mind while we are considering

Maury’s theory that currents are the result of difference of specific

gravity.

Ocean-currents, then, according to that writer, owe their existence to

the difference of specific gravity between the waters of inter-tropical

and polar regions. This difference of specific gravity he attributes to

two causes—(1) to difference as to _temperature_, (2) to difference as

to saltness. There are one or two causes of a minor nature affecting

the specific gravity of the sea, to which he alludes; but these two

determine the general result. Let us begin with the consideration of

the first of these two causes, viz.:—

_Difference of Specific Gravity resulting from Difference of

Temperature._—Maury explains his views on this point by means of an

illustration. “Let us now suppose,” he says, “that all the water within

the tropics, to the depth of one hundred fathoms, suddenly becomes oil.

The aqueous equilibrium of the planet would thereby be disturbed, and

a general system of currents and counter currents would be immediately

commenced—the oil, in an unbroken sheet on the surface, running toward

the poles, and the water, in an under current, toward the equator. The

oil is supposed, as it reaches the polar basin, to be reconverted into

water, and the water to become oil as it crosses Cancer and Capricorn,

rising to the surface in inter-tropical regions, and returning as

before” (§ 20). “Now,” he says (§ 22), “do not the cold waters of the

north, and the warm waters of the Gulf, made specifically lighter by

tropical heat, and which we see actually preserving such a system of

counter currents, hold, at least in some degree, the relation of the

supposed water and oil?”

In § 24 he calculates that at the Narrows of Bemini the difference in

weight between the volume of the Gulf-water that crosses a section of

the stream in one second, and an equal volume of water at the ocean

temperature of the latitude, supposing the two volumes to be equally

salt, is fifteen millions of pounds. Consequently the force per second

operating to propel the waters of the Gulf towards the pole would in

this case, he concludes, be the “equilibrating tendency due to fifteen

millions of pounds of water in the latitude of Bemini.” In §§ 511 and

512 he states that the effect of expanding the waters at the torrid

zone by heat, and of contracting the waters at the frigid zone by cold,

is to produce a set of surface-currents of warm and light water from

the equator towards the poles, and another set of under currents of

cooler and heavy water from the poles towards the equator. (See also to

the same effect §§ 513, 514, 896.)

There can be no doubt that his conclusion is that the waters in

inter-tropical regions are expanded by heat, while those in polar

regions are contracted by cold, and that this tends to produce a

surface current from the equator to the poles, and an under current

from the poles to the equator.

“We shall now consider his second great cause of ocean currents, viz.:—

_Difference of Specific Gravity resulting from Difference in Degree of

Saltness._—Maury maintains, and that correctly, that saltness increases

the density of water—that, other things being equal, the saltest water

is the densest. He suggests “that one of the purposes which, in the

grand design, it was probably intended to accomplish by having the sea

salt and not fresh, was to impart to its waters the forces and powers

necessary to make their circulation complete” (§ 495).

Now it is perfectly obvious that if difference in saltness is to

co-operate with difference in temperature in the production of

ocean-currents, the saltest waters, and consequently the densest, must

be in the polar regions, and the waters least salt, and consequently

lightest, must be in equatorial and inter-tropical regions. Were the

saltest waters at the equator, and the freshest at the poles, it would

tend to neutralize the effect due to heat, and, instead of producing

a current, would simply tend to prevent the existence of the currents

which otherwise would result from difference of temperature.

A very considerable portion of his work, however, is devoted to proving

that the waters of equatorial and inter-tropical regions are salter

and heavier than those of the polar regions; and yet, notwithstanding

this, he endeavours to show that this difference in respect to saltness

between the waters of the equatorial and the polar regions is one of

the chief causes, if not the chief cause, of ocean-currents. In fact,

it is for this special end that so much labour is bestowed in proving

that the saltest water is in the equatorial and inter-tropical regions,

and the freshest in the polar.

“In the present state of our knowledge,” he says, “concerning this

wonderful phenomenon (for the Gulf-stream is one of the most marvellous

things in the ocean) we can do little more than conjecture. But we have

two causes in operation which we may safely assume are among those

concerned in producing the Gulf-stream. One of these is the increased

saltness of its water after the trade-winds have been supplied with

vapour from it, be it much or little; and the other is the diminished

quantum of salt which the Baltic and the Northern Seas contain” (§ 37).

“Now here we have, on one side, the Caribbean Sea and Gulf of Mexico,

with their waters of brine; on the other, the great Polar Basin, the

Baltic, and the North Sea, the two latter with waters that are but

little more than brackish. In one set of these sea-basins the water is

heavy, in the other it is light. Between them the ocean intervenes; but

water is bound to seek and to maintain its level; and here, therefore,

we unmask one of the agents concerned in causing the Gulf-stream” (§

38). To the same effect see §§ 52, 522, 523, 524, 525, 526, 528, 530,

554, 556.

Lieut. Maury’s _two causes neutralize each other_. Here we have two

theories put forth regarding the cause of ocean-currents, the one

in direct opposition to the other. According to the one theory,

ocean-currents exist because the waters of equatorial regions, in

consequence of their higher temperature, are _less dense_ than the

waters of the polar regions; but according to the other theory,

ocean-currents exist because the waters of equatorial regions, in

consequence of their greater saltness, are _more dense_ than the

waters of the polar regions. If the one cause be assigned as a reason

why ocean-currents exist, then the other can be equally assigned as

a reason why they should not exist. According to both theories it is

the difference of density between the equatorial and polar waters that

gives rise to currents; but while the one theory maintains that the

equatorial waters are _lighter_ than the polar, the other holds that

they are _heavier_. Either the one theory or the other may be true,

or neither; but it is logically impossible that both of them can. Let

it be observed that it is not two currents, the one contrary to the

other, with which we have at present to do; it is not temperature

producing currents in one direction, and saltness producing currents

in the contrary direction. We have two theories regarding the origin

of currents, the one diametrically opposed to the other. The tendency

of the one cause assigned is to prevent the action of the other. If

temperature is allowed to act, it will make the inter-tropical waters

lighter than the polar, and then, according to theory, a current will

result. But if we bring saltness into play (the other cause) it will

do the reverse: it will increase the density of the inter-tropical

waters and diminish the density of the polar; and so far as it acts it

will diminish the currents produced by temperature, because it will

diminish the difference of specific gravity between the inter-tropical

and polar regions which had been previously caused by temperature. And

when the effects of saltness are as powerful as those of temperature,

the difference of specific gravity produced by temperature will be

completely effaced, or, in other words, the waters of the equatorial

and polar seas will be of the same density, and consequently no current

will exist. And so long as the two causes continue in action, no

current can arise, unless the energy of the one cause should happen to

exceed that of the other; and even then a current will only exist to

the extent by which the strength of the one exceeds that of the other.

The contrary nature of the two theories will be better seen by

considering the way in which it is supposed that difference in saltness

is produced and acts as a cause.

If there is a constant current resulting from the difference in

saltness between the equatorial and polar waters, then there must be a

cause which maintains this difference. The current is simply the effort

to restore the equilibrium lost by the difference; and the current

would very soon do this, and then all motion would cease, were there

not a constantly operating cause maintaining the disturbance. What,

then, according to Maury, is the cause of this disturbance, or, in

other words, what is it that keeps the equatorial waters salter than

the polar?

The agencies in operation are stated by him to be heat, radiation,

evaporation, precipitation, and secretion of solid matter in the form

of shells, &c. The two most important, however, are evaporation and

precipitation.

The trade-winds enter the equatorial regions as relatively dry winds

thirsting for vapour; consequently they absorb far more moisture than

they give out; and the result is that in inter-tropical regions,

evaporation is much in excess of precipitation; and as fresh water only

is taken up, the salt being left behind, the process, of course, tends

to increase the saltness of the inter-tropical seas. Again, in polar

and extra-tropical regions the reverse is the case; precipitation is in

excess of evaporation. This tends in turn to diminish the saltness of

the waters of those regions. (See on these points §§ 31, 33, 34, 37,

179, 517, 526, and 552.)

In the system of circulation produced by difference of temperature,

as we have already seen, the surface-currents flow from the equator

to the poles, and the under or return currents from the poles to the

equator; but in the system produced by difference of saltness, the

surface currents flow from the poles to the equator, and the return

under currents from the equator to the poles. That the surface currents

produced by difference of saltness flow from the poles to the equator,

Maury thinks is evident for the two following reasons:—

(1) As evaporation is in excess of precipitation in inter-tropical

regions, more water is taken off the surface of the ocean in those

regions than falls upon it in the form of rain. This excess of water

falls in the form of rain on temperate and polar regions, where,

consequently, precipitation is in excess of evaporation. The lifting

of the water off the equatorial regions and its deposit on the polar

tend to lower the level of the ocean in equatorial regions and to raise

the level in polar; consequently, in order to restore the level of

the ocean, the surface water at the polar regions flows towards the

equatorial regions.

(2) As the water taken up at the equator is fresh, and the salt

is left behind, the ocean, in inter-tropical regions, is thus made

saltier and consequently denser. This dense water, therefore, sinks

and passes away as an under current. This water, evaporated from

inter-tropical regions, falls as fresh and lighter water in temperate

and polar regions; and therefore not only is the level of the ocean

raised, but the waters are made lighter. Hence, in order to restore

equilibrium, the waters in temperate and polar regions will flow as

a surface current towards the equator. Under currents will flow from

the equator to the poles, and surface or upper currents from the poles

to the equator. Difference in temperature and difference in saltness,

therefore, in every respect tend to produce opposite effects.

That the above is a fair representation of the way in which Maury

supposes difference in saltness to act as a cause in the production of

ocean-currents will appear from the following quotations:—

“In those regions, as in the trade-wind region, where evaporation is

in excess of precipitation, the general level of this supposed sea

would be altered, and immediately as much water as is carried off by

evaporation would commence to flow in from north and south toward the

trade-wind or evaporation region, to restore the level” (§ 509). “On

the other hand, the winds have taken this vapour, borne it off to the

extra-tropical regions, and precipitated it, we will suppose, where

precipitation is in excess of evaporation. Here is another alteration

of sea-level, by elevation instead of by depression; and hence we

have the motive power for a _surface current from each pole towards

the equator_, the object of which is only to supply the demand for

evaporation in the trade-wind regions” (§ 510).

The above result would follow, supposing the ocean to be fresh. He then

proceeds to consider an additional result that follows in consequence

of the saltness of the ocean.

“Let evaporation now commence in the trade-wind region, as it was

supposed to do in the case of the freshwater seas, and as it actually

goes on in nature—and what takes place? Why a lowering of the sea-level

as before. But as the vapour of salt water is fresh, or nearly so,

fresh water only is taken up from the ocean; that which remains behind

is therefore more salt. Thus, while the level is lowered in the salt

sea, the equilibrium is destroyed because of the saltness of the water;

for the water that remains after evaporation takes place is, on account

of the solid matter held in solution, specifically heavier than it was

before any portion of it was converted into vapour” (§ 517).

“The vapour is taken from the surface-water; the surface-water thereby

becomes more salt, and, under certain conditions, heavier. When it

becomes heavier, it sinks; and hence we have, due to the salts of the

sea, a vertical circulation, namely, a descent of heavier—because

salter and cooler—water from the surface, and an ascent of water that

is lighter—because it is not so salt—from the depths below” (§ 518).

In section 519 he goes on to show that this vapour removed from the

inter-tropical region is precipitated in the polar regions, where

precipitation is in excess of evaporation. “In the precipitating

regions, therefore, the level is destroyed, as before explained, by

elevation, and in the evaporating regions by depression; which, as

already stated, gives rise to a system of _surface_ currents, moved by

gravity alone, from the _poles towards the equator_” (§ 520).

“This fresh water being emptied into the Polar Sea and agitated by the

winds, becomes mixed with the salt; but as the agitation of the sea by

the winds is supposed to extend to no great depth, it is only the upper

layer of salt water, and that to a moderate depth, which becomes mixed

with the fresh. The specific gravity of this upper layer, therefore, is

diminished just as much as the specific gravity of the sea-water in the

evaporating regions was increased. _And thus we have a surface current

of saltish water from the poles towards the equator, and an under

current of water salter and heavier from the equator to the poles_” (§

522).

“This property of saltness imparts to the waters of the ocean another

peculiarity, by which the sea is still better adapted for the

regulation of climates, and it is this: by evaporating fresh water from

the salt in the tropics, the surface water becomes heavier than the

average of sea-water. This heavy water is also warm water; it sinks,

and being a good retainer, but a bad conductor, of heat, this water

is employed in transporting through _under currents_ heat for the

mitigation of climates in far distant regions” (§ 526).

“For instance, let us suppose the waters in a certain part of the

torrid zone to be 90°, but by reason of the fresh water which has been

taken from them in a state of vapour, and consequently, by reason of

the proportionate increase of salts, these waters are heavier than

waters that may be cooler, but not so salt. This being the case, the

tendency would be for this warm but salt and heavy water to flow off as

an _under current towards the polar or some other regions of lighter

water_” (§ 554).

That Maury supposes the warm water at the equator to flow to the polar

regions as an under current is further evident from the fact that he

maintains that the climate of the arctic regions is mitigated by a warm

under current, which comes from the equatorial regions, and passes up

through Davis Straits (see §§ 534−544).

The question now suggests itself: to which of these two antagonistic

causes does Maury really suppose ocean-currents must be referred?

Whether does he suppose, difference in temperature or difference in

saltness, to be the real cause? I have been unable to find anything

from which we can reasonably conclude that he prefers the one cause

to the other. It would seem that he regards both as real causes, and

that he has failed to perceive that the one is destructive of the

other. But it is difficult to conceive how he could believe that the

sea in equatorial regions, by virtue of its higher temperature, is

lighter than the sea in polar regions, while at the same time it _is

not_ lighter but heavier, in consequence of its greater saltness—how

he could believe that the warm water at the equator flows to the poles

as an upper current, and the cold water at the poles to the equator

as an _under_ current, while at the same time the warm water at the

equator does not flow to the poles as a surface current, nor the cold

water at the poles to the equator as an under current, but the reverse.

And yet, unless these absolute impossibilities be possible, how can an

ocean-current be the result of both causes?

The only explanation of the matter appears to be that Maury has failed

to perceive the contradictory nature of his two theories. This fact is

particularly seen when he comes to apply his two theories to the case

of the Gulf-stream. He maintains, as has already been stated, that

the waters of the Gulf-stream are salter than the waters of the sea

through which they flow (see §§ 3, 28, 29, 30, 34, and several other

places). And he states, as we have already seen (see p. 104), that the

existence of the Gulf-stream is due principally to the difference of

density of the water of the Caribbean Sea and the Gulf of Mexico as

compared with that of the great Polar Basin and the North Sea. There

can be no doubt whatever that it is the _density_ of the waters of the

Gulf-stream at its fountain-head, the Gulf of Mexico, resulting from

its superior saltness, and the deficiency of density of the waters in

polar regions and the North Sea, &c., that is here considered to be

unmasked as one of the agents. If this be a cause of the motion of the

Gulf-stream, how then can the difference of temperature between the

waters of inter-tropical and polar regions assist as a cause? This

difference of temperature will simply tend to undo all that has been

done by difference of saltness: for it will tend to make the waters

of the Gulf of Mexico lighter, and the waters of the polar regions

heavier. But Maury maintains, as we have seen, that this difference of

temperature is also a cause, which shows that he does not perceive the

contradiction.

This is still further apparent. He holds, as stated, that “the waters

of the Gulf-stream are salter than the waters of the sea through which

they flow,” and that this excess in saltness, by making the water

heavier, is a cause of the motion of the stream. But he maintains that,

notwithstanding the effect which greater saltness has in increasing

the density of the waters of the Gulf-stream, yet, owing to their

higher temperature, they are actually lighter than the water through

which they flow; and as a proof that this is the case, he adduces the

fact that the surface of the Gulf-stream is roof-shaped (§§ 39−41),

which it could not be were its waters not actually lighter than the

waters through which the stream flows. So it turns out that, in

contradiction to what he had already stated, it is the lesser density

of the waters of the Gulf-stream that is the real cause of their

motion. The greater saltness of the waters, to which he attributes so

much, can in no way be regarded as a cause of motion. Its effect, so

far as it goes, is to stop the motion of the stream rather than to

assist it.

But, again, although he asserts that difference of saltness and

difference of temperature are both causes of ocean-currents, yet he

appears actually to admit that temperature and saltness neutralize each

other so as to prevent change in the specific gravity of the ocean, as

will be seen from the following quotation:—

“It is the trade-winds, then, which prevent the thermal and specific

gravity curves from conforming with each other in inter-tropical seas.

The water they suck up is fresh water; and the salt it contained, being

left behind, is just sufficient to counterbalance, by its weight, the

effect of thermal dilatation upon the specific gravity of sea-water

between the parallels of 34° north and south. As we go from 34° to the

equator, the water grows warmer and expands. It would become lighter;

but the trade-winds, by taking up vapour without salt, make the water

salter, and therefore heavier. The conclusion is, the proportion of

salt in sea-water, its expansibility between 62° and 82°, and the

thirst of the trade-winds for vapour are, where they blow, so balanced

as to produce _perfect compensation_; and a more beautiful compensation

cannot, it appears to me, be found in the mechanism of the universe

than that which we have here stumbled upon. It is a triple adjustment;

the power of the sun to expand, the power of the winds to evaporate,

and the quantity of salts in the sea—these are so proportioned and

adjusted that when both the wind and the sun have each played with

its forces upon the inter-tropical waters of the ocean, _the residuum

of heat and of salt should be just such as to balance each other in

their effects; and so the aqueous equilibrium of the torrid zone is

preserved_” (§ 436, eleventh edition).

“Between 35° or 40° and the equator evaporation is in excess of

precipitation; and though, as we approach the equator on either side

from these parallels, the solar ray warms and expands the surface-water

of the sea, the winds, by the vapour they carry off, and the salt they

leave behind, _prevent it from making that water lighter_” (§ 437,

eleventh edition).

“Philosophers have admired the relations between the size of the earth,

the force of gravity, and the strength of fibre in the flower-stalks of

plants; but how much more exquisite is the system of counterpoises and

adjustments here presented between the sea and its salts, the winds and

the heat of the sun!” (§ 438, eleventh edition).

How can this be reconciled with all that precedes regarding

ocean-currents being the result of difference of specific gravity

caused by a difference of temperature and difference of saltness? Here

is a distinct recognition of the fact that difference in saltness,

instead of producing currents, tends rather to prevent the existence of

currents, by counteracting the effects of difference in temperature.

And so effectually does it do this, that for 40°, or nearly 3,000

miles, on each side of the equator there is absolutely no difference in

the specific gravity of the ocean, and consequently nothing, either as

regards difference of temperature or difference of saltness, that can

possibly give rise to a current.

But it is evident that, if between the equator and latitude 40° the

two effects completely neutralize each other, it is not at all likely

that between latitude 40° and the poles they will not to a large extent

do the same thing. And if so, how can ocean-currents be due either

to difference in temperature or to difference in saltness, far less

to both. If there be any difference of specific gravity of the ocean

between latitude 40° and the poles, it must be only to the extent

by which the one cause has failed to neutralize the other. If, for

example, the waters in latitude 40°, by virtue of higher temperature,

are less dense than the waters in the polar regions, they can be so

only to the extent that difference in saltness has failed to neutralize

the effect of difference in temperature. And if currents result, they

can do so only to the extent that difference in saltness has thus

fallen short of being able to produce complete compensation. Maury,

after stating his views on compensation, seems to become aware of

this; but, strangely, he does not appear to perceive, or, at least, he

does not make any allusion to the fact, that all this is fatal to his

theories about ocean-currents being the combined result of differences

of temperature and of saltness. For, in opposition to all that he

had previously advanced regarding the difficulty of finding a cause

sufficiently powerful to account for such currents as the Gulf-stream,

and the great importance that difference in saltness had in their

production, he now begins to maintain that so great is the influence

of difference in temperature that difference in saltness, and a number

of other compensating causes are actually necessary to prevent the

ocean-currents from becoming too powerful.

“If all the inter-tropical heat of the sun,” he says, “were to pass

into the seas upon which it falls, simply raising the temperature of

their waters, it would create a thermo-dynamical force in the ocean

capable of transporting water scalding hot from the torrid zone, and

spreading it while still in the tepid state around the poles.... Now,

suppose there were no trade-winds to evaporate and to counteract the

dynamical force of the sun, this hot and light water, by becoming

hotter and lighter, would flow off in currents with almost mill-tail

velocity towards the poles, covering the intervening sea with a mantle

of warmth as a garment. The cool and heavy water of the polar basin,

coming out as under currents, would flow equatorially with equal

velocity.”

“Thus two antagonistic forces are unmasked, and, being unmasked, we

discover in them a most exquisite adjustment—a compensation—by which

the dynamical forces that reside in the sunbeam and the trade-wind

are made to counterbalance each other, by which the climates of

inter-tropical seas are regulated, and by which the set, force, and

volume of oceanic currents are measured” (§§ 437 and 438, eleventh

edition).

CHAPTER VII.

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC

CIRCULATION.—LIEUT. MAURY’S THEORY (_continued_).

Methods of determining the Question.—The Force resulting from

Difference of Specific Gravity.—Sir John Herschel’s Estimate

of the Force.—Maximum Density of Sea-Water.—Rate of Decrease

of Temperature of Ocean at Equator.—-The actual Amount of

Force resulting from Difference of Specific Gravity.—M.

Dubuat’s Experiments.

_How the Question may be Determined._—Whether the circulation of the

ocean is due to difference in specific gravity or not may be determined

in three ways: viz. (1) by direct experiment; (2) by ascertaining the

absolute amount of _force_ acting on the water to produce motion, in

virtue of difference of specific gravity, and thereafter comparing it

with the force which has been shown by experiment to be necessary to

the production of sensible motion; or (3) by determining the greatest

possible amount of _work_ which gravity can perform on the waters in

virtue of difference of specific gravity, and then ascertaining if the

work of gravity does or does not equal the work of the resistances in

the required motion. But Maury has not adopted either of these methods.

_The Force resulting from Difference of Specific Gravity._—I shall

consider first whether the force resulting from difference of specific

gravity be sufficient to account for the motion of ocean-currents.

The inadequacy of this cause has been so clearly shown by Sir John

Herschel, that one might expect that little else would be required than

simply to quote his words on the subject, which are as follows:—

“First, then, if there were no atmosphere, there would be no

Gulf-stream, or any other considerable ocean-current (as distinguished

from a mere surface-drift) whatever. By the action of the sun’s rays,

the _surface_ of the ocean becomes _most_ heated, and the heated water

will, therefore, neither directly tend to _ascend_ (which it could

not do without leaving the sea) nor to _descend_, which it cannot do,

being rendered buoyant, nor to move laterally, no lateral impulse being

given, and which it could only do by reason of a general declivity

of surface, the dilated portion occupying a higher level. Let us see

what this declivity would amount to. The equatorial surface-water

has a temperature of 84°. At 7,200 feet deep the temperature is 39°,

the level of which temperature rises to the surface in latitude 56°.

Taking the dilatability of sea-water to be the same as that of fresh, a

uniformly progressive increase of temperature, from 39° to 84° Fahr.,

would dilate a column of 7,200 feet by 10 feet, to which height,

therefore, above the spheroid of equilibrium (or above the sea-level in

lat. 56°), the equatorial surface is actually raised by dilatation. An

arc of 56° on the earth’s surface measures 3,360 geographical miles;

so that we have a slope of 1/28th of an inch per geographical mile, or

1/32nd of an inch per statute mile for the water so raised to run down.

As the accelerating force corresponding to such a slope (of 1/10th of

a second, 0″·1) is less than one two-millionth part of gravity, we

may dismiss this as a cause capable of creating only a very trifling

surface-drift, and not worth considering, even were it in the proper

direction to form, by concentration, a current from east to west,

_which it could not be, but the very reverse_.”[55]

It is singular how any one, even though he regarded this conclusion as

but a rough approximation to the truth, could entertain the idea that

ocean-currents can be the result of difference in specific gravity.

There are one or two reasons, however, which may be given for the

above not having been generally received as conclusive. Herschel’s

calculations refer to the difference of gravity resulting from

difference of temperature; but this is only one of the causes to which

Maury appeals, and even not the one to which he most frequently refers.

He insists so strongly on the effects of difference of saltness, that

many might think that, although Herschel may have shown that difference

in specific gravity arising from difference of temperature could not

account for the motion of ocean-currents, yet nevertheless that this,

combined with the effects resulting from difference in saltness, might

be a sufficient explanation of the phenomena. Such, of course, would

not be the case with those who perceived the contradictory nature of

Maury’s two causes; but probably many read the “Physical Geography of

the Sea” without being aware that the one cause is destructive of the

other. Again, a few plausible objections, which have never received due

consideration, have been strongly urged by Maury and others against the

theory that ocean-currents can be caused by the impulses of the winds;

and probably these objections appear to militate as strongly against

this theory as Herschel’s arguments against Maury’s.

There is one trifling objection to Herschel’s result: he takes 39° as

the temperature of maximum density. This, however, as we shall see,

does not materially affect his conclusions.

Observations on the temperature of the maximum density of sea-water

have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard,

Horner, and others. No two of them have arrived at exactly the same

conclusion. This probably arises from the fact that the temperature

of maximum density depends upon the amount of salt held in solution.

No two seas, unless they are equal as to saltness, have the same

temperature of maximum density. The following Table of Despretz will

show how rapidly the temperature of both the freezing-point and of

maximum density is lowered by additional amounts of salt:—

+-----------+-----------------+------------------+

| Amount | Temperature of | Temperature of |

| of salt. | freezing-point. | Maximum density. |

+-----------+-----------------+------------------+

| | ° | ° |

| 0·000123 | −1·21 C. | + 1·19 C. |

| 0·0246 | −2·24 | − 1·69 |

| 0·0371 | −2·77 | − 4·75 |

| 0·0741 | −5·28 | −16·00 |

+-----------+-----------------+------------------+

He found the temperature of maximum density of sea-water, whose density

at 20°C. was 1·0273, to be −3°·67C. (25°·4F.), and the temperature of

freezing-point −2°·55C. (27°·4F.).[56] Somewhere between 25° and 26°

F. may therefore be regarded as the temperature of maximum density

of sea-water of average saltness. We have no reason to believe that

the ocean, from the surface to the bottom, even at the poles, is at

27°·4F., the freezing-point.

The actual slope resulting from difference of specific gravity,

as we shall presently see, does not amount to 10 feet. Herschel’s

estimate was, however, made on insufficient data, both as to the rate

of expansion of sea-water and that at which the temperature of the

ocean at the equator decreases from the surface downwards. We are

happily now in the possession of data for determining with tolerable

accuracy the amount of slope due to difference of temperature between

the equatorial and polar seas. The rate of expansion of sea-water

from 0°C. to 100°C. has been experimentally determined by Professor

Muncke, of Heidelberg.[57] The valuable reports of Captain Nares, of

H.M.S. _Challenger_, lately published by the Admiralty, give the rate

at which the temperature of the Atlantic at the equator decreases

from the surface downwards. These observations show clearly that the

super-heating effect of the sun’s rays does not extend to any great

depth. They also prove that at the equator the temperature decreases

as the depth increases so rapidly that at 60 fathoms from the surface

the temperature is 62°·4, the same as at Madeira at the same depth;

while at the depth of 150 fathoms it is only 51°, about the same as

that in the Bay of Biscay (Reports, p. 11). Here at the very outset

we have broad and important facts hostile to the theory of a flow of

water resulting from difference of temperature between the ocean in

equatorial and temperate and polar regions.

Through the kindness of Staff-Captain Evans, Hydrographer of the

Admiralty, I have been favoured with a most valuable set of serial

temperature soundings made by Captain Nares of the _Challenger_, close

to the equator, between long. 14° 49′ W. and 32° 16′ W. The following

Table represents the mean of the whole of these observations:—